There have been many attempts to deduce arterial blood pressure from the time-dependent analysis of the arterial pulse, as opposed to an amplitude-dependent analysis, which cuffs and Tonometers, etc. use. The primary advantages of a time-based blood pressure monitoring system over one based on amplitude analysis are wearer comfort and inherent calibration.
Amplitude-dependent devices have to couple to the pressure wave within the artery and they have to closely track the coupling force with which they bear down on the artery. The required partial occlusion of the artery frequently leads to distinct skin markings as well as numbness of the hand when the radial artery is monitored, which is the most commonly used site for non-invasive blood pressure monitors. In addition, if the device loses track of the force with which it bears down on the artery, either because of drastic blood pressure changes or because of signal-disrupting movements, it has to be re-calibrated. If this requires inflation of a cuff, such as is the case with the Colin Pilot unit, the wearer will experience additional discomfort.
Previous attempts to deduce blood pressure from arterial pulse time domain analysis have used the well-known fact that the propagation velocity of the arterial pulse is highly dependent on the arterial pressure. These approaches have used delay times between arterial pulses measured at different arterial sites, such as the brachial and the radial artery pulse sites, or, most commonly, have used the time delay between the QRS complex of an ECG signal and a pulse measured at an arterial pulse site. In general, such two-site approaches have only been able to track substantial changes in BP using pulse transit time (PTT) but have failed to reliably resolve small changes in BP. An example of a small change in BP that is physiologically important is Pulsus Paradoxus (PP), which is defined as the abnormally large decline in systemic arterial pressure and pulse pressure associated with inspiration, usually due to an airway obstruction such as during an asthma attack.
A further and significant complication in previous PPT measurement approaches has been the determination of the diastolic and systolic BP components. The pulse location in time has usually been determined by establishing a threshold condition near the foot of the arterial pulse, either using a simple percentage of total pulse height rule or other more sophisticated methods, such as the tangent intersection method, which is the intersection of the straight-lines drawn through the rear and the fore-fronts of the arterial pulse wave. Not surprisingly, given the fact that the threshold point is close to the diastolic pressure amplitude range, delay times obtained in this manner have correlated reasonably well with diastolic blood pressure changes. However, two-site measurement approaches have been especially deficient in the measurement of systolic blood pressure variations. This is not surprising because the heartbeat pressure pulse changes dramatically in shape and amplitude as it heads toward the arterial periphery. As a result attempts to compare the time delay evolution of certain points on the pulse measured at different arterial pulse sites, aside from foot-to foot measurements, have been difficult. The changes in pulse shape are due to a number of factors, including changes in the arterial wall material composition that affect the-wall's elastic behavior, the taper of the main arterial branch, the distribution of branch lines, and pulse reflections. The result is that the pulse steepens and contracts as it propagates.
Background of the invention can be found in the following publications, the disclosures of which are incorporated herein by reference:    1—Cooke, William H, and Convertino, Victor A, Heart Rate Variability and    Spontaneous Baroreflex Sequences: Implications for Autonomic Monitoring DuringHemorrhage, J. Trauma, Injury, Infection, and Critical Care, 5˜(4):798-805, April 2005.    2—Convertino, Victor A, Cooke, William H. Holcomb, John H, Arterial pulse pressure and its association with reduced stroke volume during progressive central hypovolemia,    J. Trauma. 2006; 61:629-634.    3—Davies J I, Band M M, Pringle S, Ogston S, Struthers A D, Peripheral blood pressure measurement is as good as applanation tonometry at predicting ascending aortic blood pressure, J. of Hypertension. 21 (3):571-576, March 2003    4—Leonetti P, Audat F, Girard A, Laude 0, Lefrere F, Elghozi J L. Stroke volume monitored by modeling flow from finger arterial pressure waves mirrors blood volume withdrawn by phlebotomy. Clin Auton Res. 2004; 14:176-181.    5—MacDonald's, Blood Flow in Arteries, 4th ed. Arnold, p. 84, 1998.    6—Anliker M et. al, Transmission characteristics of axial waves in blood vessels, J.    Biomech., 1, p 235-46, 1968